Repetitive transcranial magnetic stimulation (rTMS) at 20 Hz over motor cortex can lower the threshold for motor evoked potentials (Pascual-Leone, 1994). Although rTMS at low frequency (1 Hz) can raise thresholds for evoking visual phosphenes (Fumal, 2003), rTMS has not been used extensively in the study of the visual cortex. We sought to examine the possibility that rTMS could be used to manipulate neuronal activity in specific visual areas, just as microstimulation, cooling, and pharmacological manipulation are used in animal studies.

Recent studies in both humans and monkeys have suggested that visual area V3/V3A contributes to the perception of depth based on binocular disparity; however change in function due to intervention in this area has not been demonstrated. We examined the impact of high frequency rTMS on human depth perception by using fMRI of retinotopically-organized areas to locate the target cortical region. A random dot stereogram, gradually increasing in disparity, was presented until the subject perceived it as being nearer or farther than the fixation plane. The threshold disparity for perceiving depth was measured for different locations in the visual field before, during, and after the application of rTMS. We found a significant and prolonged decrease in the disparity threshold for depth perception following high-frequency rTMS in a limited area of the visual field contralateral to the stimulated cortex. This result establishes a direct link between depth perception and neuronal activity in human areas V2 and V3. It also suggests that high-frequency rTMS could be a valuable tool for validating the contribution of cortical areas to visual perception in humans.